compulsory purchase vs expropriation

compulsory purchase

noun
  • The power of a state to expropriate private land within its jurisdiction for public use. 

expropriation

noun
  • The act of expropriating; the surrender of a claim to private property; the act of depriving of private propriety rights. 

How often have the words compulsory purchase and expropriation occurred in a corpus of books? (source: Google Ngram Viewer )